Illumination systems are widely used in microlithography to illuminate a reticle with radiation having a desired homogeneity and pupil fill. A projection objective is then used to transfer a pattern from a reticle to a substrate by forming an image of the reticle on a layer of a photosensitive material disposed on the substrate. In general, illumination systems fall into three different classes: dioptric systems; catoptric systems; and catadioptric systems. Dioptric systems use exclusively refractive elements (e.g., lens elements) to shape radiation from a source to have desired properties at an object plane of the projection objective. Catoptric systems use exclusively reflective elements (e.g., mirror elements) to shape the radiation. Catadioptric systems use both refractive and reflective elements to shape the radiation.